Metal halide vapor discharge lamp with near molten tip electrodes



July 5, 1966 3,259,777

E- G. FRIDRICH METAL HALIDE VAPOR DISCHARGE LAMP WITH NEAR N L o no Fll M y 9, 1.961 MOLTE TIP E E T DES 2 Sheets-Sheet 1 INVENTOR.

E4 {1 M52 GFB/DE/(H BY M HIS A TTOENEY July 5, 1966 E. G. FRIDRICH METAL HALIDE VAPOR DISCHARGE LAME WITH NEAR MOLTEN TIP ELECTRODES 2 Sheets-Sheet 2 Filed May 9. 1961 WAVE LENGTH (MlLLl-M/CRONS) xmwm u hi REM lnveni'or: ELmeT' gfridvich b3 OM M? Z I-Iis Atlro neg WATTS/Cm United States Patent 3,259,777 METAL HALIDE VAPOR DISCHARGE LAMP WITH NEAR MOLTEN TIP ELECTRODES Elmer G. Fridrich, South Euclid, Ohio, assignor to General Electric Company, a corporation of New York Filed May 9, 1961, Ser. No. 108,773 Claims. (Cl. 313-184) This invention relates to high pressure metal vapor electnic discharge lamps of high brightness and high efficiency useful, for example, in signal systems and for projection purposes. More particularly, the invention relates especially to such lamps emitting white light, that is, an emission of considerable intensity in the whole spectral range of visible light. The present application is a continuation in-part of my application Serial'No. 1%,- 159, filed April M, 1961, now abandoned.

It is well known to use for these purposes high pressure gaseous discharge lamps filled with xenon the spectrum of which exhibits a strong continuum in the visible wave length range. It is, however, not possible to obtain with these lamps elficiencies of more than about 50 l./w. (lumens per watt). Moreover, extremely high current densities are required for high brightness. Higher efiiciencies up to about 70 l./w. may, as is well known, be attained with high pressure mercury lamps, but those lamps exhibit the typical mercury spectrum having a low percentage of red in spite of the continuum existent at these pressures. High brightness and a degree of continuous spectrum can be achieved only at pressures in the order of 100 or more atmospheres in capillary mercury lamps. Nor has it been possible heretofore to achieve, with other metal vapors, white light at high efficiencies because of their high chemical reactivity and the fact that those other metals require much higher temperatures than mercury to obtain sufliciently high vapor pressure.

It is characteristic of lamps of the present invention thatcontinuous radiation at high brightness levels, and the high power densities necessary for high brightness, are achieved without the necessity of maintaining either extreme of excessively high pressure or very high current densities. It has been discovered that a predominantly continuous type of radiation can be produced in a lamp containing certain halide salts, preferably gallium and indium iodides, in the vapor state and at pressures in the range of one atmosphere and above, and at power densities in the range of 75 watts per cubic centimeter and above. It is thereby possible to produce, at pressures less than 20 atmospheres, visible radiation which resembles a high temperature black body radiation spectrum. With such vapors, power densities, or input load concentrations, exceeding 8,000 watts/cm. may be maintained in a quartz envelope to thereby achieve high brightnesses. At the same time, high luminous efficiencies may be attained by virtue of the selective radiant properties of those vapors.

For its mechanical and thermal properties, quartz or fused silica is generally used as the envelope material for the prior art lamps. The wall loading, i.e., the ratio of lamp wattage input to the surface area of the discharge envelope between the electrode tips in such lamps operated without liquid cooling, usually amounts to less than 40 w./cm. (watts per square centimeter), generally only about w./cm. In that case, the temperatures of the quartz envelopes are below 900 C. which is below the temperature at which quartz devitrifies.

Therefore, in accordance with another aspect of the invention, high pressure metal vapor discharge lamps embodying the present invention are characterized by a refractory envelope of small volume containing one or more metallic iodides as the principal filling material, preferaably with the addition of a starting gas, and wherein the wall loading of the discharge envelope without liquid cooling exceeds 40 W./CII1.2. Thereby lamps are available for the first time which emit white or colored light with very high efiiciencies.

White light is produced by virtue of the fact that lamps embodying the invention are thermally highly loaded. The lamps have, when compared with conventional lamps of equal wattage input, a discharge envelope of a small volume so that, thereby, a high input concentration is attained. To that end, the internal tube diameter is preferably kept less than about one centimeter, for example in the range of 1-6 mm. With higher loadings, temperature and pressure will rise. High pressures and high temperatures, however, are a prerequisite condition for the production of a continuum approximating black body radiation. In order to prevent any decrease in vapor pressure, the value of which, as is well known, is determined by the coolest place in the discharge envelope, the dead space in the lamp, i.e., particularly the space behind the electrodes, is kept as small as possible.

Those drawbacks normally associated with high temperature and hitherto considered as unavoidable, as for example sputtering of the electrodes and devitrification of the quartz generally used as the envelope material, may, as demonstrated by the present invention, be prevented. With iodides present in the discharge envelope the well known iodine cycle is brought about in the lamp with an especially high degree of effectiveness. The iodine vapor combines with the sputtered electrode material deposited on the cooler envelope walls to form a volatile compound which decomposes again above and on the hot electrodes; this process is recurrent so that blackening of the envelope wall is effectively prevented and stability of the electrodes is maintained. Also, excessive devitrification of the envelope which would cause its destruction may, as already mentioned, be prevented under certain circumstances. As is known, devitrification of quartz occurs at temperatures above 900 C. Above 1000 C. there is again a temperature range in which no devitrification occurs. By employing wall loadings such that operating temperatures lie either below 900 C. or above 1000 C. and below the softening point, devitrification is minimized.

Only by the concurrence of both the above-mentioned conditions, i.e., the high wall loading and the iodide filling, acting together, is it possible to produce the hitherto unknown type of lamp which embodies the present invention.

The invention will be better understood by reference to the following description of preferred embodiments and to the accompanying drawing wherein:

FIG. 1 illustrates in schematic. fashion, and on an enlarged scale, a metal iodine arc lamp embodying the invention for the purpose of demonstrating some of the applicable principles;

FIGS. 2a is a side view, partly sectioned, of a high pressure lamp embodying the invention; FIG. 2b is a transverse section of the same lamp; FIG. 20 is a fragmentary view illustrating the original pointed form of the electrode;

FIG. 3a is a side view, partly sectioned, of a relatively short very high pressure tipless lamp embodying the invention, and FIG. 3b is a transverse section thereof;

FIG. 4 is a side view, partly sectioned, of another more elongated tipless lamp;

FIG. 5 is an elevation, in section, of still another lamp embodying the invention;

FIG. 6 is a diagram of a typical spectral curve of a lamp embodying the invention; and

FIG. 7 is a graph showing the increase in brightness and efiiciency with respect to specific loading of a typical lamp.

In a metal halide lamp according to the invention, an envelope and electrode configuration is required which permits the plasma to extend to the envelope walls so that the entire inner surface of the arc chamber may be swept by activated (atomic) iodine produced by the discharge. This requirement entails, in general, a tubu lar discharge chamber with relatively small electrodes at opposite ends. A typical configuration is illustrated in schematic form by lamp 1 of FIG. 1 wherein a vitreous envelope 2 provides an elongated tubular discharge chamber bounded by the inner wall 3. Small pin or rod-like tungstenelectrodes 4 project into the arc chamber and operate with molten ends 5 which assume a generally spherical form as a result of surface tension. The ionizable medium within the discharge chamber consists of a suitable metal iodide and, preferably, also an inert ionizable starting gas.

The light emitting volume of such a lamp in general exhibits two zones. The central zone, which may be referred to as the core of the arc, is generally a high current region wherein the radiation consists mainly of the atomic lines typical of the metallic element or elements of the iodide salt involved. In the drawing, the core is indicated by the heavy dotted lines within the boundary 6 and extending between the electrodes. With the molten ball-point electrodes which have been illustrated, the core of the arc engulfs the molten ball-points 5, 5. Beyond and surrounding the central core is a glowing region which appears as a luminous aureole. The aureole is indicated by the lighter lines 7 and the radiation therein is largely in the visible range and exhibits a continuum. With increasing density or pressure of the metal iodide vapor, the emission from the aureole tends to outweigh or overpower that from the central portion or core of the arc with the result that the percentage of radiation in the form of line spectra including ultraviolet decreases while the proportion in the form of a continuum increases. The glowing aureole is a region of activated iodine and it extends substantially out to the inner wall or boundary 3 of the arc chamber. As illustrated in the drawing, the aureole extends even to the regions 8 to the rear of the electrodes and surrounds the electrode inleads. If molten ball-point electrodes are not used, then small rod-like tungsten electrodes are used whose tips operate at a temperature close to the melting point of tungsten. With these, the core of the arc does not engulf the electrode tips quite as effectively but nevertheless it has been observed that the glowing aureole surrounding the core may yet extend to the rear of the electrodes so that the entire inside surface of the arc chamber is swept by activated iodine. The electrode and envelope configuration schematically illustrated in FIG. 1 is thus typical of metal iodide arc lamps embodying the invention.

As an illustration of the characteristics of the metal iodide arcs, a typical example illustrating the behavior to be expected of the other metals is gallium. Although it has a low melting point, it boils at 1983 C. whereas its tri-iodide boils at 349 C. In a lamp having a quartz envelope with plain tungsten electrodes and a rare gas for starting, a wide variety of effects can be achieved by varying the geometry of the lamp, the power input, and the dosage or quantity of gallium iodide.

Lamps containing this salt, and operated in accordance with the invention, progress through the familiar warmup stages before leveling off at steady operation. Started at room temperature, they may briefly show a violet flash as the electrodes warm up. This is caused by the vaporization of the iodide which may have condensed there when the lamp cooled down previously. The violet flash immediately fades as the gallium iodide condenses out on the bulb wall. Shortly thereafter, as the entire lamp begins to warm up, the violet again reappears and be comes more and more intense as the vapor pressure of gallium iodide increases. Up to a certain point the radiation of the arc is almost all line spectrum except for the radiation from the incandescent electrodes. In this region the lamp is an efficient radiator of the 4032 and 4172 Angstrom lines along with lesser amounts of ultra-violet, visible and infrared lines. A lamp having an electrode gap of 8 mm. would be operating in this mode at as little as 10 to 20 volts.

With the development of increasing pressure of gallium iodide, in accordance with the invention, the ultraviolet radiation begins to fall off, the gallium resonance lines begin to broaden and show a reversal, the core of the arc begins to constrict and brighten, and a continuum develops. From a projected image of the lamp it is evident that the main part of the violet is coming from the core, and the continuum is being generated in the glowing aureole surrounding the core. As the pressure increases the voltage and brightness continue to grow until both the core and the electrodes appear to be overshadowed by the continuum of the aureole. The radiation then consists mainly of a continuum with a wide, dark reversal in the violet, and four lines including a yellow, an orange and two reds.

If the concentration of gallium iodide is high and the bore of the lamp is large, the tone of the lamp will vary from an original violet-tinged white to a balanced white, and even to a less efficient yellow-orange if there is too large a bore or too much gallium iodide vapor acting as a yellow filter. In these large bore lamps the are may bow up and a two-toned efiect will persist with a whiter, brighter light escaping through the upper section of the lamp where the absorbing layer of gallium iodide vapor is thinner.

On the other hand, with a very small bore, high pressure, gallium iodide lamp operating smoothly at a high loading, the arc may attain a brightness five or more times the ultimate brightness of a concentrated incandescent tungsten filament. A white light is produced which consists mainly of a continuum with a generous amount of red, somewhat selectively favoring radiation in the visible region of the spectrum and cutting off quite sharply at the violet end.

By virtue of the high temperatures prevailing in the discharge envelope it is possible to excite and to ionize in the discharge even dilficultly vaporizable metals such as, by way of example, indium, gallium and thallium and mixtures thereof, in lamps made according to the invention. Besides the spectral lines which appear partly already in self-absorption, a strong continuum occurs, whichmeans that the discharge in these lamps exhibits an obvious high pressure characteristic. Such lamps emit, if suitable metals are chosen, a white light with very high efiiciency of more than l./w. The continuum may extend also in the ultraviolet spectral range so that such lamps filled with suitable metal vapor may be used as ultraviolet radiation sources.

A strong continuum may also be obtained with iodides of other difficultly vaporizable metals such as tin, antimony, sodium, titanium and cadmium. In some cases, mercury iodide has been used in admixture with the iodides of the difiicultly vaporizable metals. Used by itself, mercury iodide does not yield a continuous spectrum.

It has been determined that suitable wall loadings, in Watts per unit of area of inner envelope wall between the electrode tips, lie in the range between 40 and 1000 or even 1200 w./cm. preferably between about and 400 or 500 w./cm. The input load concentration is more than 75 W./cm. and may be as much as 15,000 W./ cm. desirably between 1000 or 2000 and 8000 w./cm. The specific loading is preferably in the range of 50 to 1000 watts per centimeter of arc gap length, and current density in the range of 5 to or more amperes per square centimeter of envelope cross section. It is genera'lly desirable that the iodine be present in the lamp in excess of the amount which will combine stoichiometrical- Iy with the metal of any metallic iodide in the envelope when it is introduced as an iodine compound. The lamp may also desirably contain several metals as iodine salts. Eflicient radiation, particularly in wavelengths longer than ultraviolet, with high-temperature-stable, colored vapors is obtained by employing amounts of iodide salts preferably in the range of 1X l gram to 1X10 gram molecular weight per cm. of internal lamp length. Preferably, all the iodide is vaporized during normal operation of the lamp.

The operating pressure of the gases and vapors in the discharge envelope lies above about one atmosphere, preferably between 2 and 8 atmospheres; it may, however, be somewhat higher. The discharge envelope contains as a basic gas, serving as the igniting or starting means, one or several rare gases, such as argon, preferably xenon at :a filling pressure of more than mm. of Hg. The discharge envelope is preferably tubular with an electrode spacing equal to or greater than the inner diameter of the envelope.

Instead of quartz, the envelope may be composed of other high-temperature-resistant light-pervi'ous material, as for example, sapphire (aluminum oxide). The wall thickness of envelopes of quartz is, according to the preesnt invention, more than 2 mm, preferably 3 to 5 mm.; it is consequently, comparatively thick. In the case of particularly small diameters the wall thickness can be as great as, or greater than, the inner diameter of the discharge envelope. By means of surface enlargement the envelope wall is kept at temperatures at which quartz still has adequate mechanical strength. Liquid cooling may, as already mentioned, be omitted in lamps made according to the invention; if, however, quartz is used as the envelope material forced air cooling may be advantageous, particularly at loadings in excess of about 400- 600 w./cm.

The new form of electric discharge lamp utilizing a metal iodide discharge medium is characterized by the effective use of the iodine-tungsten regenerative cycle to prevent envelope blackening and to maintain the electrodes stable in shape. A compatible combination of new electrode design, lamp envelope configuration, and vapor composition in accordance with the invention results in a radiant energy source useful for its high brightness, high efiiciency or selective radiation properties. Other qualities are small size and are steadiness or optical stability.

It has been proposed in the past to use a halogen additive as a regenerative getter or clean-up agent in electric discharge lamps. While such lamps purport to have a regenerative cycle for the sake of preventing envelope blackening by restoring vaporized or sputtered metal to the electrode, usually the various halogens were considered equivalent and little or no discrimination shown in the choice among them. Furthermore, there was failure to recognize the necessity for other conditions in the ab sence of which rapid erosion and change of shape of the electrodes will result. There have also been many proposals to use halide salt vapors to impart the spectral colors of the metal component of the salt tothe arc. However, such proposals have been limited on the one hand either to low pressure, low loading applications or, on the other hand, to somewhat higher pressure discharges in which no regenerative cycle was provided to compensate for electrode sputtering or vaporization. In other instances, metal halide salts were chosen which would decompose and deposit metal on the electrodes, forming alloys or compounds therewith unless an excess of one of the more reactive halogens were included in the atmosphere of the lamp, but such excess in turn would cause rapid erosion of the electrodes. Thus, for one reason or another, the prior art lamps of this kind failed to achieve sufficient life or maintenance to be useful. By

maintenance is to be understood the ratio of light output at the stated interval of life to the initial light output.

As pointed out above, in accordance with the present invention I have found that a regenerative cycle concurrently providing a clean bulb wall and stable electrodes can be achieved in a discharge lamp or device utilizing a metal halide discharge medium providing an electrode and bulb wall configuration or geometry is observed which permits the entire inner surface of the arc chamber to he exposed to the activating etfects of the arc. By way of further explanation, it is believed that the regenerative cycle whereby refractory metal, commonly tungsten, vaporized from the electrodes is cleaned up from the wall :and redeposited on the electrodes is dependent upon having dissociated or atomic iodine reaching the bulb wall in sufficient quantity. Within the plasma, the metal iodide vapor is decomposed at least in part into metal vapor and iodine vapor, and the iodine vapor further dissociated into atomic iodine. Atomic iodine diffusing out of the plasma recombines at the walls with tungsten deposited thereon forming tungsten iodide vapor which diffuses back towards the electrodes. At the electrodes, the tungsten iodide vapor is decomposed into tungsten which redeposits on the electrodes and into iodine which again diffuses out towards the walls to repeat the process. To have clear envelope walls throughout a long operating life, the envelope configuration and the electrode size and disposition must perm-it the entire inner surface of the envelope, including the regions to the rear of the electrodes, to be swept by dissociated or active iodine. This requirement entails several concomitant conditions affecting the envelope, the electrodes, and the discharge medium, that is, the particular choice of metal halide.

Firstly the requirement in regard to the envelope may be stated simply as being that the inner boundary or envelope walls must be such that, in operation, the arc chamber is substantially completely filled by the arc and the glowing aureole which surrounds it, or in general by the plasma. In low pressure lamps the plasma extends substantially to the envelope walls. If the current or the pressure or both are increased, the discharge constricts and may no longer extend to the walls and this is the condition usually sought in high pressure lamps. Discharges in which the plasma extends to the walls are commonly described as wall-stabilized, whereas constricted discharges may be electrode stabilized or stabilized by other means.

Generally speaking, lamps according to the invention are wall-stabilized or operate in a manner equivalent to wall-stabilization inasmuch as the envelpe is proportioned to surround the arc column closely. By so doing, the inner surface of the are chamber is swept by active or atomic iodine, thus assuring effective use of the iodine regenerative cycle to clean up the walls. In general, metal iodide arc lamps according to the invention utilize vitreous envelopes, such as quartz or other transparent or translucent refractory material, which define tubular, relatively slender arc chambers, more or less elongated depending upon the arc length involved. The choice of arc length will be governed by factors including the intended use of the lamp and the total light output desired. Optimum bulb configuration for any one particular lamp will depend upon the specific choice of metal iodide or metal iodide combinations for the discharge medium and such factors as the pressure and power loading or input concentration. Metal iodide arc lamps can be designed to operate in tubular bulbs with inside diameters up to two centimeters and with input concentrations as low as 50 watts per cubic centimeter of bulb volume. In that event, the lamp emits principally only the line spectrum of the metal. The benefits of the invention, namely high brightness, high efiiciency and radiation of a continuum require lamp envelopes or bulbs having an internal diameter not exceeding approximately 1 centimeter and a volume loading not less than approximately 75 watts per cubic centimeter. V

Secondly the requirement in regard to the electrodes necessitates the use of relatively small electrodes operating at very high temperatures. Preferably rod-like tungsten electrodes are used, small for their current carrying capacity, and operating with their tips either molten or close to the melting point of tungsten. The preferred electrodes are unactivated tungsten electrodes operating at such high temperatures that a molten tip or ball-point is formed as disclosed in my copendi-ng application Serial No. 57,579 filed September 21, 1960, entitled, Electric Discharge Lamp Electrode, and assigned to the same assignee as the present invention, now Patent 3,067,357. In addition to the advantages residing in the ease of fabrication and low cost of such electrodes, they have the further advantage of achieving a steady are condition with a diffused arc-electrode junction wherein the plasma engulfs and surrounds substantially completely the glowing molten ball-point. With the arc engulfing the molten tip, the entire inside surface of the arc chamber is in line of sight with at least a portion of the arc column or plasma. In other words, no part of the inside surface of the arc chamber is shadowed from the plasma, not even the regions to the rear of the electrodes surrounding the inlead. Under these conditions, the entire surface of the envelope is swept by active iodine and a clear envelope is maintained over a long operating life. By contrast, in prior art lamps utilizing relatively massive electrodes in generally spherical-shaped bulbs, the arc attaches itself to a relatively small hot spot on the front face of the electrode with the result that the region to the rear of the electrode is shielded from the arc, is not swept by active iodine, and blackens rapidly.

Thirdly the requirement in regard to the discharge medium is that the halogen or halide used be limited to iodine or an iodide. It is possible to obtain a regenerative cycle with the other more active halogens such as bromine and chlorine. In fact, with these more active halogens, a regenerative process may be more readily established and with less restrictions as to lamp geometry under certain circumstances. However, the problem arises that the electrodes suffer rapid erosion. As a result, the electrodes change their shape and the lamps fail to achieve a long life, not as a result of envelope blackening, but as a result of destruction of the electrodes. The iodine necessary for functioning of the tungsten-iodine regenerative cycle may be supplied by dissociation of the. iodide salts elected or by the addition of a relatively small quantity of free iodine consistent with electrode stability and sufficient to promote the regenerative cycle. Whether a dissociating iodide salt or an addition of free iodine is used, the chemical reactivity which appears to be due to the presence of dissociated iodine is produced in or by the action of the arc. A glowing zone beyond the core of the arc is an indication of this reactivity and in a properly designed lamp in accordance with the invention, the discharge chamber should be proportioned such that the glowing zone or aureole of the arc extends throughout substantially to the walls. The size of the arc chamber in any one particular lamp depends upon several variables including theparticular metal halide selected for the discharge medium, the inter-electrode distance, and the power loading or input concentration proposed.

In prior proposals to use halide salts or other compounds as discharge media, the choice of metallic element was based in large part upon the atomic spectrum which the element involved would add to the radiation emitted by the arc. Elements with large numbers of closely spacedspectral lines, especially throughout the visible region, were favored, probably because their emitted light approximated a continuous spectrum which is generally preferred for lighting purposes. An important consideration of the present invention is that a metal iodide be selected for the major constituent of the discharge medium which shall be compatible with the electrodes and with the envelope material during operation. With regard to electrode compatibility, the metal of the iodide salt must not react with or alloy with the electrode to the extent that its melting point is substantially lowered. With regard to envelope compatibility, the metal must not attack or deleteriously affect the envelope material.

For example, metals of the sixth to eighth groups of the old periodic system, in general, have high boiling points and/ or unstable iodides. Without extreme excesses of free iodine only very small fractions of an atmosphere of pressure of the iodides can be maintained at high temperatures. The metals otherwise plate out rapidly on molten tungsten electrodes, usually lowering their melting point drastically. Therefore, only small traces of those metals can be tolerated.

For compatibility with the envelope wall, at the high temperatures involved, the metal of the iodide should have no oxide with a free energy of formation (per atom of oxygen content basis) less than the free energy of formation of the material of the envelope wall over the temperature range involved, for example, 500-2000 K., assuming an oxide for the envelope such as silica or alumina. Otherwise, the metal will tend to react with the envelope wall, forming an oxide of the metal and silicon iodide in the case of a quartz envelope. Thus, metals such as indium, gallium and thallium have very stable iodides, and oxides which are less stable than silica, and are therefore particularly useful with envelopes of either silica or alumina. The iodides of metals such as tin, antimony, and the alkali metals such as sodium, potassium, lithium, rubidium and cesium, while useful for relatively shorter life lamps of quartz, at very high efiiciencies in some cases, may be used for longer periods in lamps having envelopes of alumina.

In lamps according to the invention the electrodes are made of tungsten and the envelope of a refractory material like quartz, or high density, polycrystalline aluminum oxide, preferably that disclosed and claimed in application Serial No. 743,829, filed June 23, 1958, of R. L. Coble. Of the iodides which fit the compatibility requirements with tungsten electrodes and quartz envelopes which are presently the most suitable materials for hightemperature lamps, the iodides of gallium, indium and thallium have proved to be the most useful. When gallium or indium iodide or a mixture of the two are used, a small amount of free iodine is desirably included in order to avoid the formation of metallic deposits on the bulb wall. With gallium and indium iodide combinations :1 whole series of colors relatively close to white may be produced.

Lamps according to the invention are characterized by the efiicient generation at high intensity of visible radiation by reason of secondary light emission in the glowing aureole resulting from absorption of ultraviolet radiation produced in the core of the arc. The glowing aureole becomes the agent responsible for the major portion of the emitted radiation. This phenomenon is exhibited particularly by gallium and indium iodides and the color of the light generated is closely related to the color of the particular iodide vapor at high temperatures. Tin and antimony iodides exhibit this property to an even greater extent than gallium and indium iodides although they are less compatible with quartz.

Thallium iodide forms a less intensely colored vapor under similar circumstances and produces a predominantly green radiation along with a less pronounced continuum. When thallium iodide salts above the monovalent state are used, no addition of free iodine is necessary to obtain the regenerative cycle. Also thallium iodide salt in the higher valent state may be combined with gallium or indium iodide in widely varying proportions to increase the green content of the emitted light and, when used in a substantial percentage, for instance as little as one part by weight thallium to three parts indium, the need for the inclusion of any free iodine is eliminated.

In consequence of the iodine cycle by means of which the adverse effects of electrode sputtering are prevented, it is possible to load the electrodes thermally higher and thereby to keep their dimensions small. The electrodes are preferably made as short thin pins of refractory metal, especially tungsten. The electrodes should suitably be so dimensioned in respect of their diameter d (in mm.) that with an operating current i (in amperes) of the lamp the ratio i/a' lies between 10 and 60, preferably at 30. At these high current densities such high temperatures occur that the tips or points of the electrodes melt in lamp operation after the first starting and become spherical shaped. These small spherical ends have proved in further operation of the lamp to be especially favorable because they do not exhibit any undesirable deformations, even after longer periods of operation. The lamp may be operated continuously with direct or alternating current. It is also quite suitable for pulsed operation and may then, as is well known, be loaded higher for a short time than in continuous operation whereby higher efficiencies are obtainable. The lamp has the great advantage that it can be operated in any position.

A practical form of metal iodide arc lamp embodying the invention is illustrated in FIGS. 2a to 2c. Lamp 11 comprises a generally tubular envelope 12 consisting of a thick-walled originally cylindrical tube of quartz. The ends of the quartz tube are pinch-sealed onto molybdenum inleads 13 having foliated inner ends 13a which form vacuum tight seals through the quartz. As a result of the pinching or pressing operation, the ends of the quartz tube assume a waist-shaped generally rectangular cross section as best seen at 14 in FIG. 2b. Short lengths 15 of tungsten wire are welded to the foliated ends of the molybdenum lead-in wires and project into the discharge chamber. The tungsten wires which form the electrodes include a tapered portion 16 which attains its smallest diameter immediately adjacent to the ball-point or spherical tip 17. The lamp is evacuated and the discharge medium introduced through a lateral exhaust tube which is subsequently tipped off as shown at 18. In the illustration, lamp 1 is exaggerated in size; in practice it may be about centimeters long with an arc chamber about 12 millimeters long and 4 millimeters in diameter.

The lamp is provided with a filling of an ionizable medium including a metallic iodide which will provide some free or dissociated iodine during operation. For example, the filling may consist of an inert gas for starting purposes, such as argon or krypton or xenon at a pressure from to 100 millimeters of mercury, for instance argon at about 40 millimeters, with a suitable metal iodide added in suflicient quantity to provide the desired operating pressure under the proposed loading or input concentration and resulting envelope temperature.

In the lamp of FIG. 2, the electrodes are intended to operate with molten ball-points. The lamp is originally constructed using tungsten wire electrodes having a tapered portion 16 ending in a pointed tip 19 as illustrated in FIG. 20. When the lamp is first operated, the pointed tip of the electrodes becomes molten; as the current is increased, the tungsten tip melts back and forms a ball of molten tungsten which gradually increases in size, as illustrated at 17 in FIG. 2a. In order to form a ball-point of a given size by melting back the electrode point, the arc gap or inter-electrode distance is unavoidably lengthened. Therefore when electrodes having relatively large ball-points are desired in conjunction with a relatively short inter-electrode gap, it may be necessary to use an electrode which has been prefabricated at least in part. Techniques for making such electrodes are described in my previously mentioned copending application entitled, Electric Discharge Lamp Electrode.

FIGS. 3a and 3b illustrate a tipless form of very high pressure metal iodide arc lamp embodying the invention. The lamp 21 is formed by joining lengths of small bore tubing 22 to the ends of a piece of thick-walled larger bore tubing 23 which forms the arc chamber. The tungsten electrodes 24 are attached in the usual fashion to molybdenum inleads 25 having foliated ends about which hermetic seals are made by heating the quartz tubes 22 and causing them to collapse. The lamp is made by first fusing the quartz tubes 22 to the central piece 23, inserting inlead-electrode assemblies into the quartz tubes 22, and thereafter heating one of the quartz tubes to cause it to collapse about the foliated portion of the inlead and form a hermetic seal. The lamp is then exhausted and the filling, consisting for instance of an inert starting gas and a metal iodide, is introduced through the other quartz tube 22; that quartz tube is then heated and collapsed about the foliated portion of its inlead to seal the lamp. By so doing, a discharge lamp having a perfectly cylindrical central body and are chamber is obtained. This construction has the advantage of eliminating the cool spot formed by the sealed tip of a lateral exhaust tube and also the optical distortion which results therefrom.

FIG. 4 illustrates a lamp 31 similar in constnutcion to lamp 21 of FIG. 2 utilizing a larger bore central quartz body 32 to the ends of which are fused smaller bore quartz tubes 33 through which the electrodes 34 are sealed. The electrodes 34 are small tungsten rods and the lamp is operated at wattage loading or input concentration such that the ends of the electrode attain a temperature close to the melting point of tungsten without actually becoming molten. In the FIG. 5 lamp, the tubular discharge envelope 35 consisting of quartz has an inner diameter of 3.5 mm. and an outer diameter of 11 mm. At the ends of the envelope there are provided respective electrodes 36 and 37 of tungsten. The electrodes 36 and 37 are connected to the current inleads 40 and 41 by means of fused-in foils 38 and 39. The electrode diameter is about 0.6 the electrode spacing is 17 mm.

The lamp is operated in series with a choke on 220 volts. Operating voltage amounts to 105 v., current to 10 amps, wattage input to 1000 w. The lamp has a positive characteristic. The input concentration of about 6000 w./cm. for cylindrical lamps is very high; the current density is about 100 A./cm. of envelope cross-section. The lamp contains as the basic gas xenon at a filling pressure of 50 mm. of Hg, and also a quantity of indium iodide. The operating pressure of the lamp is 5 atmospheres. The lamp is cooled with air.

The spectrum of the lamp is a strong continuum extending over the whole visible spectral range which is a prerequisite condition for white light with good color rendition. Luminous flux amounts to 90,000 lumens. Consequently, thelamp attains an efficiency of l./w.

While lamps of the invention are highly efficient in units of small power, it is an advantage that they can be made of larger power merely by extending the length of the small tube and correspondingly raising the operating voltage.

In the small compact lamps of this invention, the quantity or loading of metallic iodide per unit length for the type or color of radiation produced is not too closely dependent on the lamp bore or inside diameter since the opacity of the gas is deter-mined by the number of molecules traversed by the radiation rather than by the length of the path. For example, in lamps containing indium iodide, the useful quantities are in the range of about 0.1 to 1.0 milligram of indium iodide per centimeter of internal lamp length regardless of inside diameter.

In lamps having an inside diameter less than 1 centimeter and containing the lower value of metallic iodide, i.e., about 1X10 gram molecular weight per centimeter length completely in the vapor state, the atomic line radiation in the visible will have already become the smaller part of visible radiation. In this range, the integrated visible continuum will exceed the line radiation but will in general be at a peak near the resonance lines of the metal whose iodide is used, i.e., violet for gallium, blue for indium, and green with strong near ultraviolet for thallium. The line radiation decreases with increasing pressure or iodide quantity and in the upper ranges the radiation consists mainly of continuous radiation with a black body type of distribution and no discernible fine structure. The resonance line due to the metal will have reversed. The iodine recombination band may be noticeable in the analyzed spectrum and relatively faint lines due to sodium impurity may be present.

In mercury lamps it is only under extremely high pressure conditions that any semblance of black body type of radiation occurs. It is believed that in the lamp of this invention the dominant eitects are produced by utilizing the radiative properties of high-temperature-stable colored gases.

FIG. 7 shows curves of brightness B and efficiency E versus specific loading in watts per centimeter length of the arc gap between the electrodes of a typical lamp. That lamp, similar to the one shown in FIG. 3a and designed for use as a compact high brightness source of well balanced white light, had a quartz envelope 23 of 3 mm. inside diameter, 11.5 mm. outside diameter and 16 mm. chamber length. The tungsten electrodes 24 were of 0.65 mm. diameter with ball tips of 1 mm. diameter spaced apart 11.5 mm. The lamp envelope was filled with xenon at 70 m. Hg pressure, 0.39 mg. of indium fully reacted with iodide, and 0.3 mg. excess iodine.

The discontinuity of both the brightness and efiiciency curves B and E in FIG. 7 is accounted for by the fact that in the range of loading from 400 to 1000 watts/cm. of arc length the lamp was cooled by a mild draft of air sufiicient to maintain continuous operation of the lamp at 1000 watts/cm. loading without bulging of the quartz envelope. At loading below 400 watts/cm, the lamp Was operated with natural convection cooling. The curves E and B show that both efficiency and brightness increase with loading. Both the brightness and efficiency are decreased by forced cooling, as would be expected and as evidenced by the drop at 400 watts/cm. in FIG. 7. Nevertheless highest efiieiency and brightness are obtained at the highest loadings even where forced cooling is required to avoid bulging of the quartz envelope. Of course, with an envelope of material more refractory than quartz, such as alumina, forced cooling may be dispensed with to yield still higher brightness and efiiciency. The brightness readings for curve B are the average of readings taken along a straight line joining the electrodes on a projected image of the lamp during operation.

At a power consumption of 1100 watts, this lamp was 12 Angstroms) and the line at 410.1 millimicrcns (4101 Angroms).

What I claim as new and desire to secure by letters Patent of the United States is:

1. A high pressure metal vapor electric discharge lamp comprising a tubular envelope of refractory light-pervious material having an internal diameter in the range of 1 to 10 millimeters, a pair of rod-like tungsten electrodes projecting into opposite ends of said envelope, a quantity of vaporizable metallic iodide in said envelope exerting a pressure above one atmosphere at input loadings exceeding 2000 watts/cmfi, said electrodes having their tips at temperatures close tothe melting point of tungsten at said loading in order to operate with diffuse arc-electrode junctions, said iodide being of a metal yielding a substantial continuum at the operating pressure in said lamp, said envelope being proportioned for wall stabilization of the discharge with substantially the entire inner envelope wall receiving heatdirectly from the discharge in order for the iodide regenerative cycle effectively to maintain the envelope clear.

2. A lamp as defined in claim 1 containing additionally an inert starting gas.

3. A lamp as defined in claim 1 wherein the electrode tips are molten in operation.

4. A lamp as defined in claim 1 wherein the metallic iodide is that or" indium, gallium, thallium, or mixtures thereof.

5. A lamp as defined in claim 4 and containing additionally mercury iodide.

6. A lamp as defined in claim 1 wherein the envelope is of quartz and is so proportioned in size and wall thickness relative to the input loading that the temperature at the enlarged outside surface assures adequate mechanical strength despite softening of the inside surface.

7. A lamp as defined in claim 6 wherein the envelope is of quartz and is 3 to 5 millimeters in wall thickness.

8. A compact lamp as defined in claim 4 wherein the envelope is of quartz having a wall thickness not less than the inside diameter.

9. A compact lamp as defined in claim 4 wherein the envelope is of quartz 3 to 5 millimeters in wall thickness, and wherein the filling is indium iodide and an inert starting gas.

10. A compact high brightness lamp as defined in claim 9 wherein an input loading of about 6000 Watts per cubic centimeter produces a continuum with reversal and broadening of the indium lines resulting in a well balanced white light.

References Cited by the Examiner UNITED STATES PATENTS 1,025,932 5/1912 Steinmetz 313-227 X 2,697,183 12/1954 Neunhoeffer ct al. 313184 2,765,416 lO/1956 Beese et al 313-25 X 2,965,790 12/1960 Ittig et a1. 3l3217 2,982,877 5/1961 Heine-Geldern 3l3184 3,005,923 10/196l Beese 3l3-l84 DAVID J. GALVIN, Primary Examiner.

C. R. CAMPBELL, Assistant Examiner. 

1. A HIGH PRESSURE METAL VAPOR ELECTRIC DISCHARGE LAMP COMPRISING A TUBULAR ENVELOPE OF REFRACTORY LIGHT-PERVIOUS MATERIAL HAVING AN INTERNAL DIAMETER IN THE RANGE OF 1 TO 10 MILLIMETERS, A PAIR OF ROD-LIKE TUNGSTEN ELECTRODES PROJECTING INTO OPPOSITE ENDS OF SAID ENVELOPE, A QUANTITY OF VAPORIZABLE METALLIC IODIDE IN SAID ENVELOPE EXERTING A PRESSURE ABOVE ONE ATMOSPHERE AT INPUT LOADINGS EXCEEDING 2000 WATTS/CM.3, SAID ELECTRODES HAVING THEIR TIPS AT TEMPERATURES CLOSE TO THE MELTING POINT OF TUNGSTEN AT SAID LOADING IN ORDER TO OPERATE WITH DIFFUSE ARC-ELECTRODE JUNCTIONS, SAID IODIDE BEING OF A METAL YIELDING A SUBSTANTIAL CONTINUUM AT THE OPERATING PRESSURE IN SAID LAMP, SAID ENVELOPE BEING PROPORTIONAL FOR WALL STABILIZATION OF THE DISCHARGE WITH SUBSTANTIALLY THE ENTIRE INNER ENVELOPE WALL RECEIVING HEAD DIRECTLY FROM THE DISCHARGE IN ORDER FOR THE IODIDE REGENERATIVE CYCLE EFFECTIVELY TO MAINTAIN THE ENVELOPE CLEAR. 